Is Causality Bullshit?

Great Debates
1

I am not a physics major, but I had one trying to explain why my idea wouldn’t work for superluminal communication.

This was my idea using what little I know about entanglement - I was bouncing the idea off of him but he seemed to give me a BS answer.

I know that two particles can be entangled so that if you measure one, both collapse into different states. I wasn’t so clear as to if they were always opposite states or not, but even so, we have devised communication systems where as long as ANYTHING happens to a particular something it can represent data (IE a punch card).

So if we had a system set up that if such and such particle had collapsed it might represent 1 and another particle would be 0. We could set up binary logic.

He first said I would then need to send information over a standard transmission medium to tell the Point B that it had even received a transmission. But that doesn’t make any sense, can’t I just collapse another set of entangled particles to indicate a transmission was sent. I think he sort of reached his limit to argue with me there, and just said: ‘it’s causality.’

He explained to me that if you could ever communicate faster than the speed of light it would fuck up causality. He said that I’d be able to see him do something in the past and tell me not to do it in the present.

And so I said… that it didn’t make sense. I said… think about sound. If you had enough energy/amplification behind a yell, that it could reach me from a long distance I could also indicate with my arms waving to you not to yell after you already had even if I didn’t know that you already had yelled. That doesn’t mean that I am going back in time.

I continued: If we were on the other side of a wall from each other and connected to a fiber connection that wrapped around the world 10x before it fed from your camera to my TV. You can wave your hand, and I won’t know about it until a couple seconds later. That doesn’t mean I can’t knock on the wall that separates us telling you not to and stop you from waving after you already had. We would simply see light lagging to entanglement in the same way sound lags to light.

He then told me to ‘go study physics’ and walked away with an expression like I was crazy.

So I need to know if he was right. i know my invention wasn’t spot on, but it sounds like he understands it all too well either.

2

Observing the state of a particle causes the entangled particle to collapse. You can’t just “wait and see if it collapses”. You won’t know whether it has or has not collapsed until you look at it, and looking at it causes it to collapse. You can not know whether it collapsed because of entanglement with the other particle, or because of your own observation.

3

I’m same way. It’s not my area of expertise, but the claim that sending information outside of the origin’s “light-cone” is identical to time travel feels like hand-waving.

4

But what state does a particle end up in? There are only two right? The up and down spin. Does one particle always move opposite to the other? So if I collapse the state on one side don’t I just need to reverse what I find on the other? Or is it that they aren’t always in opposite spins… or rather only while they are entangled are they in opposite spins… and since neither are observed while entangled… aren’t they in both spin states simultaneously?

5

Yeah, information can’t travel faster than the speed of light, even with entangled particles. It was all explained to me in detail by my thesis advisor who’s an expert on quantum information theory, and I’d try to regurgitate that explanation for you but my laptop’s about to die and I can’t find the power cord. Anyway, it’s 1 AM here. Tomorrow, maybe?

6

I know this stuff is tough to figure out too. But Mosier’s explanation makes sense. You have a particle with unknown spin. I have a particle entangled with yours with unknown spin. If I find out what the spin of my particle is, say ‘up’, then with these particles, yours would then have ‘up’ spin also. But what are you going to do? If you find out the spin of your particle, how do you know whether that is because you looked at it, or I looked at it? Whoever looked at it first was the one to fix the spin in a known state. So the theory goes, you need to send some data at light speed to me so we can coordinate. You look at you particle, and see it had ‘up’ spin. You send me a message at light speed to look at my particle. Now I know you fixed the spin as ‘up’. If I look at the particle before I get your message, I have no way of knowing whether I fixed the spin as ‘up’, because you didn’t look at yours yet, or if you set the spin to ‘up’, because you did look at it already.

So here is my followup question. Why couldn’t you and I keep synchronized clocks that tell us when to check the particle. You will look at the particle at 12 noon, and I will look at the particle at 1 second after noon. I’ll know that the particle’s spin was fixed by you. Why doesn’t that work to give us FTL communication if we are more than 1 light second apart? Is it because the quantum state can’t be as simple as being ‘on’ or ‘off’?

7

I’m not sure what any of this has to do with “causality”.

The classic causality example is creating a wormhole that travels back in time by 5 minutes. You can create a paradox if you shoot yourself through the wormhole.

8

Say you have a pair of entangled particles. They can be in one of two states, “up” or “down”.

You keep one in your lab on earth and send the other on a rocket to the Moon, several light-seconds away.

At a predetermined time you observe your particle. There’s a 50% chance it will be up, and a 50% chance that it will be down.

At the same time, your friend on the Moon observes his particle. There’s also a 50% chance that his will be up and a 50% chance it will be down.

As far as both of you can tell, the state of your particles are completely random.

However, when you compare notes later you notice that every time you measured “up”, he measured “down”, and vice versa. The particles were always the opposite of each other.

No problem, you say, clearly when they were originally entangled one was set to be “up” and the other was set to be “down” and they preserved that information after they were separated.

However, it’s possible with a more complicated set-up (which is difficult to explain) to rule this out. Each particle is “deciding” on whether to be “up” or “down” at the moment of measurement, not before. But, they both always “pick” the opposite choice, even though they’re far enough apart that no message could pass between them.

Einstein called this “spooky action at a distance”.

However, it can’t be used to send a message because the only thing you see at each end is a random sequence of “up” and “down” measurements. It’s only when you compare notes afterward that you discover their behavior is correlated.

9

Then can’t we just measure their correlation once, and from then on know what the measured results are the next time we check without having to compare notes again? Do we have to do this every time?

10

I don’t know, but that’s what he said. I guess he thought faster than light communication meant being able to stop events from occurring that had already happened, like you said with your paradox.

11

The causality problem here seems to be about information. If FTL communication worked, you could send lottery numbers back in time.
Hmm? Where can I get some of these entangled particles?

12

The point is that you can’t SET an entangled particle to a particular state and have its partner mirror it. When you measure it, it randomly assumes a particular value. At the same time its partner assumes the opposite value. But there’s no way to use this to send a message. No matter what you do, all the person on the receiving end sees is a random sequence of states. He can’t tell if what he’s seeing is the result of your actions, or if you stopped measuring half and hour ago and he’s just getting random results from his own measurements. It’s only by comparing his results to yours that the connection emerges.

13

I think you can only do this once per particle. I’d still like to know why you can’t just have a clock to tell you when to check the particle.

14

Oh well.

Still, I think the whole idea that I could change an event I’ve seen by using FTL communication is bull. Maybe I am wrong.

15

How would you send a message anyway? Let’s for a moment forget about all the quantum stuff: I give you a letter, in which there is either a sheet with the number ‘0’ or the number ‘1’ written on it; I keep a similar letter, which contains the other number. You open your letter, see ‘0’, and know immediately that mine must contain ‘1’. This is obviously not quite the way things happen in quantum mechanics, seeing as there can be no local hidden variables, but if one wanted, one could posit some classical probability function encoding our mutual knowledge about the system – which in effect says that any one envelope contains a 0 with 50% probability, and a 1 also with 50% probability --, which ‘collapses’ upon opening any one letter, simultaneously and apparently in violation of causality no matter how far both letters are apart. Again, you open your letter, and your letter’s probability function collapses to ‘100% 0’, and you know instantly that mine has collapsed to ‘100% 1’ – apparently, you have gained information about a far removed system faster than it could have travelled to you at the speed of light. The trick is now to figure out how to send a message with this – and at least in this example, it’s easy to see that it can’t be done, as there is no way for you to influence whatever you are going to find in your letter, and hence, what I am going to find in mine. The situation in quantum mechanics is sufficiently similar that we need not go into the subtleties about hidden variables and such.

As to why information transfer with faster than light speed violates causality, a little thought experiment: Imagine you had a spaceship, and a transporter that can instantly transport you to any point within its range (it doesn’t have unlimited range, or else, you wouldn’t need a spaceship, would you? Seems obvious, yet apparently not to the writers of the latest Star Trek film…). As you fly merrily through space, you zip past another ship, and only in the rear view mirror do you realize that it was the ship of your old friend, Zarg the Destroyer (space is very small, as the writers of the latest Star Trek film knew all too well, so coincidences like that happen all the time). So you think it’d be really nice to catch up with ol’ Zarg, hop into your teleporter (say, ten minutes after you passed each other), and have yourself transported over – instantly, remember? Now, thanks to special relativity, since Zarg’s ship is moving relative to you, time on his ship is ticking a little slower – for numerical reference, say that the two ships recede from each other at about 87% of the speed of light, which means that on Zarg’s ship, clocks run half as fast as on yours; thus, you get there five minutes after you passed one another. Now, the instant you materialize, you realize that you have forgotten the DVD (of the latest Star Trek film) Zarg lent you, so you decide to quickly hop back to your ship to get it. However, the situation is perfectly symmetrical – so, from Zarg’s point of view, time on your ship is ticking half as fast, meaning that, since five minutes have elapsed on his ship, only two and a half have elapsed on yours since you passed one another! Of course, that’s awfully convenient, since now you don’t have to go digging up the DVD yourself, but can just remind your past self not to forget it, which of course means that you never will have had occasion to travel back to your ship and tell your past self… and so on.

Things work out essentially that way with every speed faster than that of light, and only below that can we be guaranteed a nice, cause-precedes-effect order to the universe; so even if c weren’t a hard and fast limit to speed in the universe, I’d advise keeping to it, or else you’re probably just gonna wind up with a dead grandfather who also happens to be your twin that took a trip to Alpha Centauri. That, or Vulcan gets blown up.

16

Alright, I was thinking about it, and maybe it’s because we shouldn’t be using the speed of light as a constant. Why can’t time be the constant?

We believe we know time isn’t constant based on atomic clocks running slower in orbit correct? But maybe that’s just gravity’s effect on clocks and not time.

The Doppler effect you postulate, time is running slower on one ship because it is moving faster correct? What if time wasn’t really running slower, just their clocks? I mean we’ve never really determined if a human out in space or at high speeds ages slower, have we? This is all based on mechanical devices, and really even if it did work on humans… humans are also in some ways mechanical and maybe this is just a physical effect and not a physics effect (if that makes sense).

If time is truly constant, there might not be any real mechanical way (even atomic way) to measure it.

So lets say in ‘true-time’ you teleport at 6:00, you should arrive at 6:00 on the other faster moving ship, even if his clocks say 5:55, then if you teleport back immediately you will arrive back at your ship at 6:00.

Mechanically the clocks might vary greatly based on gravity and speed, but the constant time might not be measurable, and is probably unaffected by these things.

Say my friend was one light-hour away in space, and I was looking at him using a large telescope. For me it might be 6:00 and for him it might be 5:30, but the constant ‘true-time’ for both of us would be the same.

What I am seeing through the telescope though is an afterimage, light radiation from one hour before. If we had some sort of instant communication set up, if he waved, and I saw it and told him, his reply would be: ‘I did that an hour ago.’ If I asked him to wave, he would do it immediately, but I wouldn’t see it for an hour.

IMO that is not breaking any laws, and from what I have read causality isn’t really a required part of any physics systems, it’s more of a moral law or implied law.

So… anyway… this causality stuff is BS, even if you can’t communicate with super-luminous speed.

17

Simply put, it doesn’t agree with experiment. High-speed elementary particles take longer to decay than when they are at rest wrt the laboratory frame, for instance.

It’s not the doppler effect (which merely accounts for a frequency shift of light - or any other wave - when source and receiver are moving relative to each other), but that’s not important. But I’m curious as to what you believe is the difference between time running slower, and things happening taking longer? In other words, if, in every test system you accelerate to high enough speeds, processes uniformly take longer to occur, then what’s the meaning in asserting that time still doesn’t run slower? Time (as far as we’re concerned) is what the clock measures, and every physical process that has some time dependency is a clock.

What’s the meaning of a ‘constant time’ that’s not measurable, and hence has no observable effects?

I’m afraid that your argument boils down to ‘suppose special relativity (in the form of the constancy of the speed of light) were violated. Then, special relativity (in the form of the causal ordering of (Minkowski-) spacetime) would be violated!’, which doesn’t exactly tell us very much…

18

Doh!:smack: It’s so obvious now that you told us the answer!

So the ‘spooky action’ is indistiquishable from random events unless you compare what happens at both ends, something that can only be done using regular old communications.

:wink: Now I think I know what went wrong with my Hieronymous machine! Have to get back to that :wink:

19

Don’t feel bad though. The first time I heard about quantum entanglement was some show on the science channel(I think it was Understanding) where they said the way it worked was you entangled a pair of particles. Then you could take the spin of one, flip it and the spin of the other pair would change regardless of how far apart they were. This of course confused me as to why you couldn’t use this to communicate faster than light and to me made no sense. Turns out that explaination is confusing at best. (Hamster’s explaination is on the nose. As far as I know the explaination on the show was just wrong.)

20

I’m not sure that you can.

Actually you are correct that gravity causes time dilation. But what is time anyway? Mechanical, chemical and atomic processes take place at specific rates. If those processes proportionately slow down as the object containing them increases in velocity, then “time”, for all intents and purposes is moving slower.

Think of it like this. The speed of light in a vacuum c is the speed limit of the universe. Nothing can travel faster. And this speed is constant for all objects in all frameworks. So space and time have to bend so that I can actually use my flashlight on a ship traveling at c.